Cardiac assistance systems provide additional cardiac output in patients who suffer from insufficient cardiac output. One type of cardiac assistance system is called a left ventricular assist device (LVAD). LVADs are auxiliary pouches intended to function as booster pumps to aid the hearts of individuals suffering from chronic congestive heart failure. This condition is frequently due to heart attacks that reduce the pumping capacity of the human heart. By boosting the capacity of such a weakened heart, individuals suffering from this condition may be allowed to again lead relatively normal, effective lives.
While various designs of LVADs have been proposed, the most promising appears to be an auxiliary pouch formed from the individual's latissimus dorsi muscle and controlled by a pacemaker. This approach avoids potential rejection problems related to the use of non-autologous materials and takes advantage of well developed pacemaker and prosthetic vascular graft technology. LVADs of this type are commonly called skeletal muscle ventricles (SMVs).
To create an SMV in a human involves exposing the left latissimus dorsi muscle and dissecting the muscle free from the subcutaneous tissues and chest wall, except for the neurovascular bundle and humeral insertion. A bipolar nerve cuff electrode is placed around the thoracodorsal nerve. The nerve lead is connected to an inactive neurostimulator, buried beneath the left rectus abdominis muscle, which innervates the exposed latissimus dorsi muscle.
Next, the left chest is opened at the fourth rib. Preferably, the fourth rib is removed to provide more space for the LVAD. Optionally, the anterior pericardium is removed between the phrenic nerves and used to cover a conically-shaped mandrel of biocompatible plastic. Mandrels used for beagles had a diameter of about 3 cm, length of about 6.5 cm and volume of about 25 ml; a mandrel suitable for forming a human SMV would need to be appropriately enlarged. After wrapping the pericardium around the mandrel it is sewn to a 5 mm thick collar of synthetic material such as woven Dacron felt placed at the base of the mandrel. The dorsal edge of the latissimus dorsi muscle is then folded longitudinally upon itself and secured by sutures, after which the -medial aspect of the latissimus dorsi muscle is wrapped around the mandrel (and over the pericardium if it was used) about 2-2.5 times with the folded edge of the muscle sewn circumferentially to the Dacron sewing ring. The SMV is then positioned subcutaneously and the wound is closed and allowed to heal for three weeks.
Following this healing period, a stimulator such as the Medtronic Model 4710 is activated to transform the fatigable Type II latissimus dorsi muscle fibers to fatigue-resistant Type I muscle fibers. Typically, 6 weeks are allowed for this stimulation period, after which the chest is again opened to connect the formed muscle pouch to the aorta. This is accomplished by first attaching sensing leads to the left ventricle. The descending thoracic aorta is exposed to allow two 12 mm ringed vascular grafts to be anastomosed to the aorta, one above the other, in end-to-side fashion.
After completion of these anastomoses, the aorta between the two graft anastomoses is at least partially ligated. The plastic mandrel is removed from within the muscle pouch and a sewing ring is used to couple the pouch to the aorta. The aorta is then at least partially ligated forcing blood flow through the newly formed SMV. Finally, the nerve lead and myocardial leads are connected to an R-wave synchronous pulse-train stimulator.
One problem such past cardiac assist systems have faced, however, is with the sewing ring. Past designs of sewing rings were fabricated from a square piece of DACRON fabric into which a hole was formed. The piece of fabric was sew to the SMV and the circulatory system such that the hole permitted the inner chamber of SMV to communicate with the circulatory system. This design, however, tended not to be adequately fixed to the muscle of the SMV. In particular, because the muscle was sutured at its edge to the flat piece of fabric, the fabric or sewing ring would be subject to shear force during the contraction of the muscle. This would cause, in time, the sewing ring and SMV to detach from one another, or rupture.